The sunlight spectrum has, on the earth, a broad wavelength band from ultraviolet rays to far infrared rays having a peak wavelength of approximately 600 nm. In order to receive such sunlight and convert it to electric energy with solar cells depending on a single energy band gap, the usable spectral range is limited and there is a limitation in terms of high efficiency photo-electric conversion. Therefore, in order to absorb a wide range sunlight spectrum, a solar battery of a photo-electric conversion structure has been proposed in which the sunlight spectrum is divided into plural sensitivity wavelength bands, and plural types of solar cell modules (or element solar cells, or solar cell layers) capable of accomplishing photo-electric conversion with high efficiency for each sensitivity wavelength band are laminated sequentially in the order of short central wavelength (large band gap) in sensitivity wavelength bands from the incidental side of sunlight.
Conventional solar batteries that have been proposed so far include the following.
(a) A solar battery in which the sunlight is divided into multiple wavelength bands through optical filter mirrors, and multiple types of independent solar cell modules suitable to respective sensitivity wavelength bands are arranged on sunlight paths, as described in N. S. Alvi, C. E. Bakus and G. W. Madesen, “Potential For Increasing the Efficiency of Photovoltaic Systems by Using Multiples Cell Concept”, Conf. Proc. 12th IEEE Photovoltaic Specialist Conference 957 (1976).
(b) A solar battery in which multiple semiconductor layers having different energy band gaps are made by sequential crystal growth on a common substrate to integrally laminate a two-layer solar cell layer.
(c) A solar battery in which multiple types of solar cell panels are individually prepared by using semiconductors of different sensitivity wavelength bands and are arranged on the optical path of sunlight.
Solar batteries of above (b) and (c) are described in A. W. Bett, F. Dimroth, G. Stollwerck, O. V. Sulima “III-V Compounds For Solar Cell Applications” Appl. Phys. A69, 119-129 (1999).
In solar batteries of above (a) to (c), a pn junction is prepared in a planar semiconductor wafer or a semiconductor layer for element solar cells constituting a laminated solar battery. The solar battery of above (a) is not free from the degradation due to the optical loss of filter-mirrors and expensive manufacturing cost. The space among multiple element solar cells is large and requires a great deal of labor in their alignment and fixation, etc.
In the solar battery of above (b), the type of semiconductors that can be crystallized and grown on one substrate is restricted due to differences of crystal structure and lattice constant, making difficult the formation of pn junctions of different band gaps and a desirable shapes. In addition, tunnel junctions are necessary so that an electric current flow between laminated solar cell layers, but the resistance of the tunnel junctions is high. Moreover, the magnitude of the photo-electric current of laminated multiple solar cell layers become uneven, with the problem that the output current of the entire solar battery is restricted by a solar cell layer having the lowest magnitude of photo-electric current.
In the solar battery of above (c), the restriction of crystal growth such as the solar cell of above (b) is eliminated, but a window which allows light of a wavelength band not absorbed by the element solar cells is necessary for the element solar cells on the incidental side of sunlight. If the number of lamination and light-receiving areas of element solar cells is increased, there are drawbacks such as the fact that the effective light-receiving area is easily decreased due to an area increase of a comb-like electrode section and positional deviation of element solar cells. As with the solar battery of above (b), since solar cells consisting of a single pn junction are laminated, the problem remains that the magnitude of output current of element solar cells is uneven, and the output of the entire solar battery is restricted by an element solar cell having a small output current.
An objective of the present invention is to eliminate above described problems and to provide a laminated solar battery capable of improving remarkably the photo-electric conversion efficiency of sunlight.